When performing medical imaging, such as computed tomography imaging, it is important to know the location, or elevation, of the patient relative to the imaging machine prior to initiating the scan. Such information is important, for example, so that an accurate image can be reconstructed from the scan data.
In at least one known table apparatus used in an imaging system, the table apparatus includes a cradle support, a base, parallel table support legs, a driving mechanism, and an encoder. A cradle is located on the cradle support and is movable on the cradle support through the gantry. Each parallel table support leg has one end pivotally connected to the cradle support and the other, opposite, end pivotally connected to the base. The pivot connections are sometimes referred to hereinafter as pivot points. The cradle is maintained in a substantially horizontal position and is movable vertically and laterally relative to the base. The driving mechanism, which may be a hydraulic or electric actuator, has one end pivotally mounted to one of the table support legs and an opposite end pivotally mounted to the base.
The encoder is coupled to one of the support legs and, in operation, generates pulses indicative of the angular orientation (theta) of the support leg with respect to the base. The output of the encoder is coupled to a control processor which is programmed to control operation of the imaging machine.
In operation, the cradle support typically is initially located at a lower position. A patient lies on a cradle, located on the cradle support. The driving mechanism then drives, or pushes, the one leg support so that the cradle support and cradle move upward (vertically) and towards (laterally) the gantry. As the angular orientation of the legs changes, the encoder generates pulses indicative of such angular orientation.
The pulses, as explained above, are supplied to the control processor. The pulses may be store in an accumulator, and once the patient is positioned at the desired elevation, the accumulated pulse count is utilized to determine the elevation of the cradle support. Particularly, such elevation is determined by the processor using the following relationship: EQU Height=R * sin (theta)+offset
where:
R is the attachment point of the cradle support (length of the table support leg); PA1 theta is the angle supplied by the encoder; and PA1 offset is an adjustment for the base and cradle offsets from the pivot points. PA1 A is the vertical elevation of the cradle support relative to its initial position, PA1 k is a predetermined, e.g., by mechanical ratio by placement of the vertical feedback support, constant, and PA1 is the vertical height of the horizontal encoder support rail relative to its initial position.
To determine the patient elevation as explained above, processor firmware resources and time are required. Of course, it is desirable to limit the amount of such required resources and time. In addition, determining patient elevation in this manner is very sensitive to measurement errors and calculation roundoffs depending on the angle of elevation because the sensitivity of the encoder is high at higher elevations and low at lower elevations. Moreover, the relationship between encoder pulses, or counts, is non-linear in that such counts are not proportional to elevation. The non-linear nature of the elevation determination adds complexity to the elevation determination and increases the possibility for errors.
It is desirable, therefore, to reduce the processor resources and time required to determine cradle support elevation prior to a scan, and to utilize methods and apparatus which are not sensitive to measurement errors and calculation roundoffs depending on cradle elevation. It also is desirable to provide methods and apparatus which utilize linear relationships to determine cradle support elevation to simplify cradle support elevation determinations and further reduce the possibility for elevation determination errors.